In the photographic processing industry, strips of photographic sheet material are commonly transported through processing machines by roller trains positioned to provide the desired transport path. It is necessary to have each of the rollers positively driven at a uniform speed to prevent slippage of the material on said rollers and to provide uniform driving force and speed as well as generally constant tension along the entire length of said strip. Several driving mechanisms have been developed heretofore, none of which are totally satisfactory. One type of mechanism employs gear trains connecting the rollers in an assembly and is complex and cumbersome, especially if the transport roller assembly is of substantial length. Another type of mechanism uses rotary crank arms, all respectively connected to the same ends of the rollers. The crank arms are connected in parallel relation by a single connecting bar to apply rotational power to only one end of the rollers. Due to play in linkages between the respective arms and the connecting bar, the arms may move out of parallel relationship with each other (especially with long roller trains and transport paths) and as a result the connecting bar may be on one side of the center of some rollers and on the other side of the center on other rollers. This condition occurs when the arms are in a substantially vertical position, and causes a reversal of rotational direction of some of the rollers which renders the drive linkage inoperative.
This invention prevents such a reversal of rotational direction by transferring the power from the top to the bottom of the transport on both ends of the rollers by the use of two sets of parallel crank arms and two connecting bars, one bar on each end of the transport rollers with each set of crank arms being is sufficiently out-of-phase relationship to each other so that when one set of crank arms is in its ambiguous vertical position the other set positively maintains the direction of rotation and substantially constant torque is applied to the rollers connected to the respective arms regardless of crank arm position.